In recent years, due to that a material having corrosion resistance effective under a high temperature and a high pressure has been developed, attention is paid to a high-pressure acid leach (HPAL) as a hydrometallurgical process of nickel oxide ore. The HPAL is different from a pyrometallurgical process which is a conventional and common metallurgical process of nickel oxide ore and has such a merit that it is advantageous regarding energy and cost since it does not include a dry process such as a reducing and drying process and is composed of a consistent wet process. That is, in the above-described HPAL, by controlling oxidation-reduction potential and a temperature of leachate in a pressurizing and leaching reactor through a leaching process, iron of major impurity is fixed to leached residue in a form of hematite (Fe2O3), nickel and cobalt can be leached selectively to iron, which results in a large merit.
For example, as the hydrometallurgical process of nickel oxide ore, a high-pressure acid leaching process utilizing an autoclave is adopted.
In a high-pressure acid leaching process for obtaining nickel/cobalt mixed sulfide, for example, as shown in FIG. 7, a pre-processing process (1), a high-pressure acid leaching process (2), a solid-liquid separating process (3), a neutralization process (4), a dezincification process (5), a sulfurization process (6), and a detoxification process (7) are included (see PTL 1, for example).
In the pre-processing process (1), material slurry with a predetermined slurry concentration including ores of 2 mm or less is prepared by cracking and classifying nickel oxide ore using crushing equipment and sieving equipment. The material slurry is supplied to the next high-pressure acid leaching process (2).
In the high-pressure acid leaching process (2), after the material slurry obtained in the pre-processing process (1) is raised in temperature and pressure in a stepwise manner in a preheater (temperature-raising and pressure-raising equipment), it is supplied to an autoclave, and sulfuric acid similarly raised in temperature and pressure is added to the material slurry in the autoclave, with stirring is performed at a temperature of 220 to 280 degrees C., valuable metals are acid-leached with high-pressure and high-temperature, and leached slurry to be obtained is lowered in temperature and pressure down to the ordinary temperature and the normal pressure in a flash vessel.
In the solid-liquid separation process (3), the leached slurry of valuable metals obtained in the leaching process (2) is separated into solid component and liquid component, and leachate (crude nickel sulfate aqueous solution) containing nickel and cobalt as the valuable metals and leached residue are obtained.
In the neutralization process (4), the leachate obtained in the solid-liquid separating process (3) is neutralized.
In the dezincification process (5), zinc is precipitated and removed as zinc sulfide by adding hydrogen sulfide gas to the leachate neutralized in the neutralization process (4).
In the sulfurization process (6), nickel/cobalt composite sulfide and nickel barren solution are obtained by adding hydrogen sulfide gas to the dezinced post-solution obtained in the dezincification process (5).
In the detoxification process (7), the leached residue generated in the solid-liquid separating process (3) and the nickel barren solution generated in the sulfurization process (6) are detoxified.
Here, in the autoclave used the high-pressure acid leaching process (2), the heated and pressurized material slurry and sulfuric acid are supplied to a first compartment partitioned into a plurality by partition walls in the autoclave, leaching is advanced while stirring is being performed by a stirrer provided in the first compartment, the slurry is transferred to a second compartment and compartments subsequent thereto through overflow or the like, so that leaching is further advanced in a similar manner.
Now, when the slurry is transferred to the next compartment within the autoclave, there is a case where the transfer is performed through a liquid flow port provided at a lower portion of the partition wall in addition to the case where the transfer is performed by overflowing an upper portion of the partition wall (see PTL2, for example).
The slurry to be overflowed and the slurry on a bottom portion of each compartment are generally different in staying time in the compartment from each other, though depending on a situation of slurry fluidity in the compartment obtained by stirring. Therefore, by transferring the overflowed slurry and the slurry on the bottom portion with good balance to the next compartment, long and short of the staying time can be balanced so that efficient operation is intended by averaging all of the staying times.
Further, as a roll of the liquid flow port not in a steady operation but when the operation is emergently stopped due to any trouble, there is such a matter that it is made possible to supply slurries in the respective compartments to a final-stage compartment in order to continue slurry discharge from the autoclave as long as possible.
The amount of discharge slurry from the autoclave is controlled by a valve provided between the autoclave and the flash vessel such that a set liquid level of the autoclave is maintained. When the slurry supply to the autoclave is stopped due to any trouble, it is necessary to close a discharge valve in order to maintain the liquid level of the autoclave, but once the discharge valve is closed fully, for opening the discharge valve again, it is necessary to lower the temperature and the pressure of the inside of the autoclave largely as compared with those in a normal operation in order to prevent equipment damage due to urgent evaporation in a discharge pipe and the flash tank.
When such a work has been performed, a long period of time is required to return the ordinary operation, which results in a large loss of an operating time. Therefore, even if the operation is emergently stopped, the slurry in the autoclave continues to be discharged at an extremely slight flow rate without fully closing the discharge valve. Liquid flow ports are provided so as to make it possible to supply slurries in the respective compartments to the final compartment through the liquid flow ports to continue to maintain the slurry discharge as long as possible at this time.
Therefore, it is an important point for designing that the sizes of the liquid flow ports are made as small as possible in the steady operation in order to maintain an overflow state for transferring the slurry through overflow and the slurry on the bottom portion to the next compartment with good balance, and the sizes of the liquid flow ports are made as large as possible for an emergency stopping time, and the sizes are properly adjusted and set according to the flow rate in a real operation or the size of the autoclave.
However, there is such a case that the balance is collapsed and slurry transfer from the liquid flow ports provided on the lower portions of the partition walls becomes dominant, so that slurry transfer through the overflow is almost gone, and such a problem arises that efficient operation is obstructed because the staying time of slurry is biased to the slurry on the bottom portion.
Since the problem causes such an effect that the overall leaching rate of the autoclave lowers, so that an excessive amount of sulfuric acid as compared with an amount equivalent to valuable metals contained in the material slurry is charged into the autoclave in order to maintain a predetermined leaching rate (ordinarily, 90 to 95%) on the operation. In order to maintain the operation, management is performed based upon the amount of free sulfuric acid (unreacted sulfuric acid remaining in the above-described leachate), and the value of the amount is ordinarily 50 to 55 g/L or so.
Further, since the leachate obtained in the solid-liquid separating process (3) is neutralized in the next neutralization process (4), the free sulfuric acid is neutralized without contributing to leaching, which is wasteful. Further, since a cost of a neutralizer for neutralization is required, there is such a problem that it is desired to reduce the amount of the free sulfuric acid as much as possible, but the current state lies in such a situation that the above-described amount of free sulfuric acid must be kept in order to maintain the predetermined leaching rate.
For example, such a technique is proposed that an optimal temperature condition is maintained without performing external heating or cooling by such an idea as maintaining the volume of reactive slurry in the first compartment within the autoclave larger than the volume of the reactive slurry in any of the downstream side compartments by 50 to 200% (see PTL 3, for example).
Further, a technique is proposed to reproduce sulfuric acid according to a composite implementation composed of two or more continuous leaching stages including a stage of leaching at the normal pressure and a stage of pressurized leaching and reduce the amount of sulfuric acid by an amount equal to the amount of added sulfuric acid (see PTL4, for example).
However, since the disclosed technique in the above-described PTL 3 targets sulfide material and the disclosed technique in the above-described PTL 4 requires the process of leaching at the normal pressure, these techniques cannot be applied to the above-described problem.
Further, in the autoclave apparatus having compartments partitioned into a plurality by the partition walls, openings for a manway used when a worker inspects inside of the autoclave in such a time as a periodical inspection are provided on respective partition walls and the above-described openings for a manway are closed.
In view of the strength of the partition wall, the liquid flow port for slurry transfer is provided in a door member for a manway together with a door of the liquid flow port for slurry transfer, and the autoclave apparatus is used during an ordinary operation in a state where the door for a manway has been closed and the door of the liquid flow port for slurry transfer has been opened.
The manway is used when the worker inspects inside of the autoclave in such a time as a periodical inspection, and since the inclination of a central bottom portion is the smallest inside of the autoclave as a foothold, the door for a manway is provided in the vicinity of the central bottom portion of the partition wall. Therefore, it is popular that the door of the liquid flow port for slurry transfer is also provided in the central bottom portion of the partition wall.